Various types of ion sources may be used to create the ions that are used in semiconductor processing equipment. For example, Bernas ion sources operate by passing current through a filament disposed in a chamber. The filament emits electrons which excite the gas that is introduced to the chamber. A magnetic field may be used to confine the path of the electrons. In certain embodiments, electrodes are also disposed on one or more walls of the chamber. These electrodes may be positively or negatively biased so as to control the position of ions and electrons, so as to increase the ion density near the center of the chamber. An extraction aperture is disposed along another side, proximate the center of the chamber, through which the ions may be extracted.
One issue associated with Bernas ion source is the life of the filament. Since the filament is exposed in the chamber, it is subject to sputtering and other phenomenon which reduce its life. In some embodiments, the life of the Bernas ion source is dictated by the life of the filament.
A second type of ion source is the indirectly heated cathode (IHC) ion source. IHC ion sources operate by supplying a current to a filament disposed behind a cathode. The filament emits thermionic electrons, which are accelerated toward and heat the cathode, in turn causing the cathode to emit electrons into the chamber of the ion source. Since the filament is protected by the cathode, its life may be extended relative to the Bernas ion source. The cathode is disposed at one end of a chamber. A repeller is typically disposed on the end of the chamber opposite the cathode. The cathode and repeller may be biased so as to repel the electrons, directing them back toward the center of the chamber. In some embodiments, a magnetic field is used to further confine the electrons within the chamber.
In certain embodiments, electrodes are also disposed on one or more side walls of the chamber. These electrodes may be positively or negatively biased so as to control the position of ions and electrons, so as to increase the ion density near the center of the chamber. An extraction aperture is disposed along another side, proximate the center of the chamber, through which the ions may be extracted.
One issue associated with IHC ion sources is that the cathode may have a limited lifetime. The cathode is subjected to bombardment from electrons on its back surface, and by positively charged ions on its front surface. The ion bombardment results in sputtering, which causes erosion of the cathode. In many embodiments, the life of the IHC ion source is dictated by the life of the cathode. In certain embodiments, chemical vapor deposition from the plasma may cause the negatively charged cathode to become electrically connected to the grounded walls of the ion source, causing failure of the ion source.
Therefore, an ion source that has increased life may be beneficial. Further, it would be advantageous if the ion source experienced less sputtering on the components used for electron generation.